The present invention relates to a ball end mill having one or more generally arcuate cutting edges arranged at a forward end of an end mill body. More specifically, the present invention relates to a ball end mill capable of providing sufficient rigidity of the end mill body ensuring the cutting performance of the cutting edges, while controlling the chattering of the end mill body.
A conventional ball end mill having two cutting edges is illustrated in FIGS. 21 and 22. The ball end mill in these figures comprises an end mill body 1 in the form of a circular cylinder. The end mill body 1 has a hemishepherical forward end with which a pair of chip pockets 2 and 3 are formed. A pair of tip mounting seats 4 and 5 are formed at a wall surfaces 2a and 3a of the chip pockets 2 and 3 which face in the cutting (rotational) direction of the end mill body 1. A pair of throw-away tips 6 and 7 (hereinafter referred to as "main tip 6" and "second tip 7") are removably mounted respectively to the tip mounting seats 4 and 5 by respective clamp bolts 15.
The main tip 6 and the second tip 7 are made of cemented carbide and formed in the shape of a plane plate having an uniform thickness. Main cutting edges 10 are formed symmetrically about the center of the main tip 6 at intersections between the rake face 6a and the side faces 6b of the main tip 6.
The main cutting edge 10 consists of an arcuate cutting edge 8 having substantially a 1/4 arc and a linear cutting edge 9 rearwardly extending from a rear end of the arc cutting edge 8. Further, the cutting face 6a is shaped in a convex curved surface on the opposite ends of the rake face 6a. A convex curved portion 8a is formed at a edgeline of each convex curved surface. The second tip 7 has substantially the same structural features as the main tip 6. The second tip 7 is formed with a second cutting edges 13 formed at intersections between the rake face 7a and the side faces 7b of the second tip 6 (refer to FIG. 24). The second cutting edge 13 consists of an arcuate cutting edge 11 having substantially a 1/4 arc and a linear cutting edge 12 rearwardly extending from a rear end of the arc cutting edge 11. The second tip is distinguished from the main tip 6 by planar sections 14 which are formed on the side face of the second tip 7 adjacent the front end of the arcuate cutting edge 11.
As shown in FIGS. 23 through 25, the main tip 6 and second tip 7 are arranged such that the front end of the main cutting edge 10 of the main tip 6 is located at the rotational center P.sub.0 of the end mill when viewed from the front end, and such that the front end of the second cutting edge 13 of the second tip 7 is located at the rear position remote from the rotational center P.sub.0, and both tips 6 and 7 are removably mounted respectively to the end mill body 1 by respective clamp bolts 15. Further, the axial rake angles A of the main cutting edge 10 and the second cutting edge 13 are set at a positive angles. According to this construction, when viewed from the front end, the main cutting edge 10 and sub cutting edge 13 are in the shape of a circular arc drawn about the centers P.sub.1 and P.sub.2 located at the side position of mounting faces 6c and 7c of the tips 6 and 7.
Another example for a conventional end mill, there is known as a single edged ball end mill as illustrated in FIGS. 26 through 29. The ball end mill in these figures has substantially same structural features as the above mentioned ball end mill except for having only a main tip 6. Therefore, the same numerals are put on the same components as the above mentioned ball end mill to avoid repetition of the description.
In such a ball end mill as well, the main tip 6 is arranged such that the front end of the main cutting edge 10 of the main tip 10 is located at the rotational center P.sub.0 of the end mill when viewed from front end, and is removably mounted to the end mill body 1 by the clamp bolt 15. Further, the axial rake angle .gamma..sub.A of the main cutting edge 10 is set at a positive angle. According this construction, as shown in FIG. 29, the main cutting edge 10 is in the shape of a circular arc drawn about the center P.sub.1 located at the side position of the mounting face 6c when viewed from front end. According to the above mentioned ball end mills, in order to set the axial rake angles .gamma..sub.A at a positive angle, the wall surface of the tip pockets 2 and 3 facing toward the cutting direction and bottom faces of the mounting seats 4 and 5 must be formed such that these faces are inclined counter to the cutting direction of the ball end mill. For this reason, the thickness behind the chip pockets 2 and 3 gradually decrease toward the rear end of the end mill. As a result, the rigidity of the end mill body 1 decreases, so the chattering of the end mill body occurs often. This sometimes causes the ball end mill to break at the side of the main cutting edge 10 which bears a high cutting load.
In order to solve the above drawback, it may be suggested to set both axial rake angles .gamma..sub.A of the main cutting edge 10 and sub cutting edge 13 at negative angles so as to increase the thickness of the portion behind the chip pockets 2 and 3. However, in such a ball end mill, the cutting performance of the end mill decreases since the cutting resistance affects to the main cutting edge 10 and second cutting edge 13 increase. This results in various drawbacks such as increase of consumption of electric power and frequency of breakage of cutting edge.